1. Field of the Invention
The field of the present invention relates to finishing machines of the types including a tractor and a screed towed by such tractor. More particularly, the field of the invention relates to asphalt finishing machines having screeds of the free floating or self-leveling type.
2. The Prior Art
As is known to those skilled in the art, such screeds are towed behind the tractor by a pair of draft arms, the latter being pivotally connected at their forward ends to the frame of the tractor. As the screed is pulled into the material deposited on the subgrade by the forwardly moving tractor, the screed will automatically seek a level where the bottom of the screed becomes parallel to the direction of pull of the pivot points of the leveling arms. This type of screed operation depends upon time, distance and mix stability; properly utilized, such a screed can produce a smooth and level mat over a rough sub-base.
A finishing machine of the type under consideration includes a hopper at its forward end which must be replenished from time to time from a truck carrying the supply of asphalt. In order to refill the hopper, forward motion of the finishing machine is temporarily arrested to permit the truck to come into position for dumping its contents into the hopper of the finishing machine. When the finishing machine is stopped, the full weight of the stationary screed comes to rest on the incompletely compacted asphalt material. Under these conditions, the screed will sink into the freshly laid material thereby creating an unsightly mark on the surface of the roadway.
Quite often, the screed will sink into the mat to such an extent that an undesirable depression will be formed in the surface of the roadway mat. When forward motion of the paving machine is resumed, the attitude of the screed will cause it to rise slightly above its desired free floating level and this will in turn cause a hump to be formed on the roadway mat adjacent the aforementioned mark or depression. This undesirable result obtains because the screed overshoots the desired level before it achieves self-leveling thus aggravating the aforedescribed situation which causes formation of a hump on the surface of the roadway mat.
As is also known to those skilled in the art, screeds of the type under consideration are often provided with automatic screed control systems. These automatic systems control the screed level to establish grade lines and for matching adjacent mats or gutters. For a detailed description of such a system, reference may be had to U.S. Pat. No. 3,285,148, in the name of J. M. Munyon, and assigned to the assignee of the present invention. These automatic systems control the angle of attack of the screed, as by raising or lowering the pivot points at the forward ends of the draft arms, and use various methods of obtaining grade references, such as a reference stringline to grade, a joint matching shoe referencing to an adjacent mat or a long ski which moves with the finisher and acts as a traveling straight edge.
With the use of a finishing machine having a free floating screed and an automatic screed control system as just referred to, the screed will still settle into the freshly laid mat when forward motion of the paver is arrested for refilling the hopper. When forward motion of the finishing machine is resumed, the sensing system forming part of the automatic screed control system will sense that the screed is somewhat below the desired level of the roadway mat. This sensing action will cause the automatic control system to raise the tow points of the draft arms until the screed regains the desired level; however, such action will usually aggravate formation of the undesired hump referred to above, because the automatic screed control system will cause the screed to overshoot to some extent before it attains the predetermined level.